Conventionally, a station (hereinafter referred to as a STA) connected to an access point (hereinafter referred to as an AP) of a wireless LAN system has been disconnected from the AP when moving out of the service area where the STA is out of communication with the AP. Therefore, the STA starts to scan for a next connectable AP. In IEEE 802.11 Std., two types of scan methods by which a STA scans for an AP are prepared (see non-patent document 1).
In passive scanning described in “11.1.3.1 Passive scanning” in IEEE 802.11 Std., a STA monitors to a beacon which is transmitted from an AP for each wireless channel during a certain period, and thereby scans for the existence of an AP. Furthermore, in active scanning described in “11.1.3.2 Active scanning” in IEEE 802.11 Std., a STA transmits a frame called a probe request for each wireless channel, and, when a response called a probe response is sent back from an AP, the STA recognizes the existence of an AP.
As an example of an AP scan, a sequence of an AP scan by active scanning is shown in FIGS. 1 and 2.
In FIG. 1, a STA disconnected from the AP transmits probe requests sequentially to channel number (Ch#) 1 to Ch#3, for example. In FIG. 1, a connectable AP in Ch#3 sends a probe response, and the STA starts processing to establish connection to the AP using Ch#3. On the other hand, in FIG. 2, none of Ch#1 to Ch#3 includes a connectable AP, and so the STA repeatedly transmits probe requests to each of the channels until the STA connects to an AP.
As described above, in the existing AP scan method adopted by a STA, a STA which is not connected to an AP continues scanning until the STA finds an AP, and immediately starts connection processing when the STA finds a connectable AP.
FIG. 3 is a flowchart showing the existing procedure by which a STA scans for an AP to which the STA can be connected. Incidentally, in FIG. 3, the initial value of a channel number n is 0, and nmax is the maximum value of the channel number.
First, when the STA is disconnected from the AP (step 101), the STA decides between active scanning and passive scanning (step 102).
When active scanning is performed, the STA adds 1 to n, and transmits probe requests sequentially to Ch#1 to Ch#nmax (step 103 to step 105). When a connectable AP exists in any one of Ch#1 to Ch#nmax and a probe response is sent back from the AP (“Yes” in step 104), the STA starts processing to establish connection to the AP using the channel (step 111).
Furthermore, when no connectable AP is found to exist after the STA transmits the probe requests sequentially to Ch#1 to Ch#nmax, the STA resets n to 0 (step 106), goes back to step 103, and repeats the AP scan until the STA connects to an AP.
When passive scanning is performed, the STA adds 1 to n, and sequentially scans Ch#1 to Ch#nmax (step 107 to step 109). When a connectable AP exists in any one of Ch#1 to Ch#nmax (“Yes” in step 108), the STA starts processing to establish connection to the AP using the channel (step 111).
Furthermore, when no connectable AP is found to exist after the STA sequentially scans Ch#1 to Ch#nmax, the STA resets n to 0 (step 110), goes back to step 107, and repeats the AP scan until the STA connects to an AP.    Non-patent document 1: IEEE 802.11 Std. “11.1.3.1 Passive scanning”, “11.1.3.2 Active scanning”